1. Field of the Invention
The rate of liquid droplet absorption by a capillary or porous substrate is measured by an optical electronic system.
2. Description of Related Art
Spontaneous penetration of viscous and viscoelastic fluids into pores is observed in various natural and physiological processes and has numerous application in medicine and biomedical engineering, cosmetics and personal care, oil recovery and agriculture, catalysis and separations paper and fiber industries etc. This process may be very fast. Fast absorption is commonly studied by high speed photography. The best commercial instrument known provides up to 350 drop images per second. This is known as the xe2x80x9cDrop Shape Analysis System, DSA 10,xe2x80x9d manufactured by Krxc3xcss of Charlotte, N.C.
The use of light beams for measuring and testing are common in the art. S. Hunt, U.S. Pat. No. 2,545,281, issued Mar. 13, 1951, tests liquid absorbency characteristics of materials during successive stages of absorption. The device uses a horizontal surface, a liquid dispenser, a timing device, a light beam and photo cell for determining liquid dispersal. The light beam and timer are used to measure the rate the liquid takes to pass through the dispenser to the material tested. B. Haley, U.S. Pat. No. 2,868,062, issued Jan. 13, 1959, tests absorption using optical means. A roller traverses a ramp and energizes a light source focused on the ramp. Photoelectric means receive light reflected from a liquid treated porous sheet on the ramp while the amount of reflect light with time is kept by a recording means. J. Banner, U.S. Pat. No. 4,720,636, issued Jan. 19, 1988, uses a light beam to detect liquid presence. Phototransistors respond to either the shadow or reflection from the liquid to furnish information to a comparator circuit and an integrator circuit. Thompson et al, U.S. Pat. No. 4,628,468, issued Dec. 9, 1986, teach light beam use for predicting pore-dependent physical properties of microporous solids. The apparatus includes a detector, recording units, a computer and memory. Fischer et al, U.S. Pat. No. 3,807,875, issued Apr. 30, 1974, teach a densitometry apparatus using a light beam passed through a sample to measure chemical concentrations, sedimentation rates, absorption, and light scattering phenomena.
To better understand the present invention, the art has recognized parameters and characteristics for absorption of materials and knowledge obtained by the expensive and time-consuming photographic procedures incorporated here. These are demonstrated by photographs and graphs. The concepts are inherently applicable to the present invention. To demonstrate these typical viscous and visco-elastic fluids, distilled water, water/glycerin mixture (50/50), aqueous solutions of polyethyleneoxide (PEO) with molecular mass of 4xc3x97106, and polyacrylamide (PAM) with molecular mass of 11xc3x97106, were used. Stainless steel capillaries of 0.46 mm and 0.65 mm diameters, glass capillaries of 0.65 mm inner diameter, and sugar cubes were used as absorbents.
The surface tension as measured by the drop weight method was 72 nN/m and 65 mN/m for water and for water-glycerin mixture, respectively. In the whole range of studied concentrations, the surface tension of PAM solutions was the same as that of water. The surface tension of PEO solutions depended on the concentration and decreased from 72 mN/m to 62 mN/m as the PEO concentration increased from zero to 100 ppm.
The rheological behavior of the fluids was analyzed by using a co-axial cylinder viscosimeter to measure the shear viscosity and the MicroRheotester developed for testing polymer solutions under stretching. Bazilevsky A. V., Entov V. M., Rozhkov A. N., xe2x80x9cLiquid filament microrheometer and some of it""s three applicationsxe2x80x9d. The Golden Jubilee meeting of the British Society of Rheology and Third European Rheology Conference, 1990, Edinburgh, UK.
The dependencies of the shear viscosity versus the shear rate are presented in FIG. 4. In the process of absorption, the high shear rates are typical. The shear rate can be estimated as xcex93xe2x88x92U/R where U is the velocity of fluid penetration and R is the capillary radius. Taking Uxcx9c15 cm/s and R=0.3 mm we get the estimate of the shear rate as 500 sxe2x88x921. At such large shear rates, xcex93100 sxe2x88x921, the shear viscosity of PEO solutions was practically the same as that of water, 1 mPa-s. At the same time, PAM solutions show a non-Newtonian behavior, especially in the range of shear rates between 100 sxe2x88x921 and 500 sxe2x88x921. As the shear rate increases further, the viscosity of PAM solutions tends to a certain limiting value. For 200 ppm solution, this value is approximated by the viscosity of water, while for 500-1000 ppm solutions, the viscosity of water/glycerin mixture is a suitable estimate.
In MicroRheotester, the dynamics of thinning of liquid filaments is analyzed to characterize the Theological behavior of fluids in extensional flows. It is assumed that the fluid flow can be described by the upper convected Maxwell model. In addition to the shear viscosity inherent in simple liquids, this model involves another physical parameter, the relaxation time xcex. The latter is of the order of a time interval during which the polymer coil assumes its spheroidal shape after deformations. As shown in FIGS. 4 and 5, all polymer solutions showed well pronounced viscoelastic properties.
FIGS. 6-8 show video frames taken during droplet absorption The time intervals between the first, second and third images are about one second. The process takes about 10 ms between the third and fifth images. These images confirm that the droplet remains spherical until it touches the substrate. As soon as the contact is established and the absorption begins, we see the bridge formation preceding instant droplet detachment. Although the time intervals are very small, they are sufficient for stress relaxation. That is why the contact line is held pinned to the capillary brim, and, similarly to the traditional scheme of droplet formation, the detachment is caused by breakdown of the liquid bridge. While the process of bridge rupture is almost unaffected by the substrate properties, there is a striking difference between absorption of water and absorption of polymer solutions. The first four frames in FIGS. 7 and 8 are almost identical and the time intervals between them are comparable. The time intervals between the first, second and third images of FIG. 7 are about one second, between the last fourxe2x80x942-3 milliseconds. In FIG. 8 the time intervals are: between the first, second and third imagesxe2x80x942-3 secs, between the third, fourth and fifthxe2x80x942-3 ms, between the fifth and sixthxe2x80x9414-16 ms, and 0.5 sec between the sixth and the eighth images. The polymer additives do not affect significantly the hydrodynamics of neck formation. They do affect the droplet snap off at the late stages when the neck transforms into a thin filament. Almost cylindrical filaments were detected for water droplets as well, but it disappears swiftly. The stability of the filament formed by a PEO solution reflects the effect inherent with macromolecular solutions: during the bridge thinning the coils are stretched thus forming a bundle of xe2x80x9cpinsxe2x80x9d stabilizing the bridge. The filament lifetime is an order of magnitude longer than the time of neck formation. Thus, the fluid rheology influences the process of droplet detachment significantly. An analysis of the kinetics of filament thinning can be used for the determination of Theological parameters of the fluid.
The dynamics of droplet absorption is quantitatively characterized by FIGS. 3, 9, and 10. A typical record of the optical signals specifying the effect of fluid rheology is presented in FIG. 3. As shown in FIG. 9, the initial droplet size does not influence the process of absorption. Thus the rate of absorption is controlled by pore level effects. The flow rates of water, water/glycerin and 200 ppm PAM solutions differ insignificantly. The average flow rate agrees with the data for water obtained earlier by standard techniques. The kinetics of absorption of polymer solutions is controlled by and is a function of the relaxation time.
In FIG. 10 the mass concentration is specified for each point. As seen from FIGS. 9-10, the velocity reduction is pronounced for the polymer solutions of concentration greater than 200 ppm. The concentration of 200 ppm corresponds to the overlap concentration for PAM solutions when the polymeric coils become occasionally entangled. Above the overlap concentration, the solutions are xe2x80x9cjelly;xe2x80x9d they manifest the visible effect of normal stresses in shearing flows of viscoelastic fluids.
The above examples show that the method is capable of distinguishing the Theological behavior of tested fluids. The examples reveal a significant difference between absorption of viscous and viscoelastic fluids. It is expected that the method is sensitive enough to catch the characteristic features of absorption of fluids with a more complex Theological behavior, such as biofluids.
The invention is to a new technique applicable to various substrates. Instead of high speed photography, a millisecond resolution of the change in volume/radius of the droplet remaining atop the substrate is monitored by an optical measuring system.
The optical measuring system consists of a light source, a photo diode with a multiplier connected to a computer through an Analog/Digital converter. A droplet of liquid is emitted from a syringe by a dosing screw. A signal is processed to determine the droplet volume and the time the droplet absorption begins.
Measuring the time interval between the maximum droplet size and disappearance of the droplet we get the time of droplet absorption. A syringexe2x80x94substrate assembly is placed between a light source and an optical sensor. A droplet is emitted. As soon as it touches the substrate, the fluid begins to penetrate inside the pores. The luminous flux from the light propagating through the gap between the needle of the syringe and the substrate is measured during the process of absorption. An analysis of the dynamics of droplet absorption is reduced to an interpretation of the optical signal. The intensity of the optical signal depends on the cross sectional area of the liquid located in the gap. The intensity of the signal is practically independent of the optical properties of liquids. The signal seen on the screen of the monitor and stored in the computer characterizes the amount of light blocked. Thus, the signal decreases as the droplet penetrates into the substrate. In the simplest version the time interval between the droplet formation and its disappearance due to complete absorption are measured. Since the initial droplet suspended at the syringe is almost spherical, the droplet volume is easily calibrated into the optical signal with a high accuracy. The amount of the absorbed liquid is calculated as the difference between the initial volume and the volume of the residual droplet attached to the syringe. The rate of absorption is quantified by the average volumetric flow rate Q=volume absorbed/time to absorb, or, in the case of capillary, by the average fluid velocity U given by U=Q/xcexR2, where R is the pore radius.
The method can be used for a wide range of different porous substrates. Among them solid and soft membranes, personal care and biomedical fibrous materials, fibers, rough surfaces, yarns, etc. The technique can also be utilized in the microfabrication applications for quantitative analyses of the wettability, permeability and sorption capacity of structured substrates, including chips for microfluidic devices, nano and micro electromechanical systems, chips for protein recognition, and the like. The method can also be used for screening the effect of the presence, absence, or amount of protein in treated fluids.